MRI system having implantable device safety features

ABSTRACT

One embodiment of the present invention relates to a telemetry device which is in communication with an MRI system, and which is operable to communicate with an implantable medical device (IMD). The telemetry device includes a controller and telemetry circuitry. In one embodiment, the telemetry device is operable to automatically detect the presence of an IMD, and determine if the IMD is in an MRI-safe mode. If the IMD is not in an MRI-safe mode, the telemetry device can initiate processing to prevent the MRI system from conducting an MRI scan while the IMD in not in an MRI-safe mode.

BACKGROUND OF THE INVENTION

The present invention relates generally to implantable medical devices(“IMDs”) and magnetic resonance imaging (“MRI”) systems, and moreparticularly to systems, devices and methods for rendering IMDs moresafe in the presence of strong electro-magnetic interference, such asthose produced by (“MRI”) systems.

IMDs can be used to provide a number of different medical therapies topatients. For example, therapeutic IMDs can include pacemakers,implantable cardioverter defibrillators (“ICDs”), blood pumps, drugdelivery devices, neurostimulating devices, and the like. Some of themost common IMDs include pacemakers and ICDs (collectively referred toas cardiac rhythm management (“CRM”) devices), which are used to controlthe heart rate when heart rhythm disorders occur.

Magnetic resonance imaging (MRI) is an efficient technique used in thediagnosis of many disorders, including neurological and cardiacabnormalities and other diseases. MRI has achieved prominence in boththe research and clinical arenas. It provides a non-invasive method forexamining internal body structures and functions. Because MRI has becomesuch a useful diagnostic tool, it now is used extensively in hospitalsand clinics around the world.

As one skilled in the art will appreciate, MRI systems produce extensiveelectromagnetic fields during operation. In particular, MRI systemsgenerally produce (and utilize) three types of electromagneticfields: 1) a strong static magnetic field; 2) a time-varying gradientfield; and 3) a radio frequency (RF) field which consists of RF pulsesused to produce an image. The static field produced by most MRI systemshas a magnetic induction ranging from about 0.5 to about 1.5 T. Thefrequency of the RF field used for imaging is related to the magnitudeof the static magnetic field, and, for current-generation MRI systems,the frequency of the RF field ranges from about 6.4 to about 64 MHz. Thetime-varying gradient field is used in MRI for spatial encoding, andtypically has a frequency in the Kilohertz range.

These strong electromagnetic fields produced by MRI systems can causeproblems for implantable medical devices, such as CRM devices. Forexample, the static magnetic field can affect the magneticallycontrolled (reed) switch that prevents inappropriate programming of apulse generator (“PG”), and in some cases, it can saturate the core ofinductive switching power supplies, causing difficulties for someimplantable device power supplies. Further, the time-varying gradientfield can generate significant voltage in CRM device leads, which cancause false cardiac event sensing. Finally, some tests have shown thatthe RF field produced in MRI systems can cause CRM device heating, andvoltage generation in the CRM device circuitry and leads. Of particularconcern are the MR-induced voltages, which potentially can inhibitpacing and/or ICD defibrillation, or which can induce excessively rapidpacing and/or inappropriate ICD defibrillation shocks. Both of thesemalfunctions can be life-threatening events. Indeed, some deaths havebeen reported for patients with implanted CRM systems who wereinadvertently subjected to MRI scans. As a result, both the U.S. Foodand Drug Administration (FDA) and many pacemaker manufacturers haveissued warnings against pacemaker recipients undergoing MRIs.

Also, as one skilled in the art will appreciate, the adverse effects ofMRI fields are not limited to CRM devices. MRI fields can adverselyaffect other IMDs, as well. Thus, a need exists for systems, methods,and/or devices that can mitigate the hazards associated with using CRMdevices and other IMDs in an MRI environment.

BRIEF SUMMARY OF THE INVENTION

One embodiment of the present invention relates to a telemetry devicewhich is in communication with an MRI system, and which is operable tocommunicate with an implantable medical device (IMD). The telemetrydevice includes a controller and telemetry circuitry. In one embodiment,the telemetry device is operable to automatically detect the presence ofan IMD, and determine if the IMD is in an MRI-safe mode. If the IMD isnot in an MRI-safe mode, the telemetry device can initiate processing toprevent the MRI system from conducting an MRI scan while the IMD in notin an MRI-safe mode.

In one embodiment of the invention, the processing to prevent the MRIsystem from conducting an MRI scan can include the telemetry generatingan alarm message when the IMD is not in an MRI-safe mode. In anotherembodiment, the processing to prevent the MRI system from conducting anMRI scan can include the telemetry device transmitting a command to theMRI system instructing it to not conduct an MRI scan while the IMD isnot in an MRI-safe mode.

In yet another embodiment, if the IMD is not in an MRI-safe mode, theprocessing to prevent the MRI system from conducting an MRI scan caninclude the telemetry device transmitting a command to the IMD,instructing it to switch to an MRI-safe mode prior to the MRI systembeginning an MRI scan. In addition, in another embodiment, the telemetrydevice also can transmit a signal to the MRI system to begin the MRIscan after the telemetry device receives confirmation from the IMD thatthe IMD is in an MRI-safe mode. In yet another embodiment, after the MRIscan is complete, the telemetry device then can transmit a command tothe IMD instructing the IMD to switch back to a non-MRI-safe mode.

In one embodiment, the telemetry device can be an IMD programmer or anIMD repeater device. In addition, the telemetry device can be configuredseparate from the MRI system, or the telemetry device can be built intoMRI system.

In another embodiment of the invention, the IMD can be operable todetect a magnetic field produced by the MRI system. In accordance withthis embodiment, the telemetry device can be configured to receive acommand from the IMD instructing the telemetry device that an IMD ispresent and not in an MRI-safe mode. The telemetry device then cancommunicate this information to the MRI system, preventing the MRIsystem from conducting an MRI scan.

In still other embodiments, the present invention can relate to an MRIsystem including the foregoing telemetry device.

A more complete understanding of the present invention may be derived byreferring to the detailed description of preferred embodiments andclaims when considered in connection with the figures.

BRIEF DESCRIPTION OF THE DRAWINGS

In the Figures, similar components and/or features may have the samereference label. Further, various components of the same type may bedistinguished by following the reference label with a second label thatdistinguishes among the similar components. If only the first referencelabel is used in the specification, the description is applicable to anyone of the similar components having the same first reference labelirrespective of the second reference label.

FIG. 1 is a diagram showing the relationship between an implantablemedical device and an MRI system in accordance with one embodiment ofthe present invention;

FIG. 2 is a block diagram of one embodiment of an implantable medicaldevice that can be used in the present invention;

FIG. 3 is a block diagram of one embodiment of a telemetry system thatcan be used in the present invention; and

FIGS. 4-10 are flow charts illustrating different embodiments of methodsof the present invention.

DETAILED DESCRIPTION OF THE INVENTION

The present invention relates generally to magnetic resonance imaging(“MRI”) systems and implantable medical devices (“IMDs”), and moreparticularly to systems, devices and methods for rendering IMDs moresafe in the presence of strong electro-magnetic interference (“EMI”),such as those produced by MRI systems. In accordance with at least someembodiments, the present invention relates to IMDs that can beprogrammed to alter their operational modes in the presence ofelectro-magnetic interference to prevent damage to the IMD and/or thepatient. As discussed in more detail below, the IMDs can be programmedfrom an external programming device, or the IMDs can be configured toautomatically change operational modes in the presence of the EMI.

As used herein, the term electro-magnetic interference (“EMI”) can referto any EMI, such as static magnetic fields, gradient magnetic fields,and/or radio frequency (“RF”) fields generated by an MRI system, or anyother electro-magnetic fields or interference that can be generated byany number of different sources, such as metal detectors, radiotransmitters, cellular phones, microwave generators, electronic articlesurveillance systems, etc. Thus, the present invention can be used torender IMDs more safe in the presence of any EMI and is not limited toany particular EMI environment. As one skilled in the art willappreciate, however, operating IMDs during MRI scans or at leastrecognizing the presence of the IMDs prior to an MRI scan is ofparticular interest to health care providers. Thus, for this reason, andfor ease of presentation, the present invention will be discussed withreference to MRI systems. The present invention, however, is not limitedto an MRI environment.

Also, as discussed above, some embodiments of the invention relate toswitching operational modes of the IMDs to render them more safe in thepresence of EMI, and in particular, MRI fields. In these embodiments,the IMDs are switched from a “normal” operational mode to an “MRI-safe”operation mode. As one skilled in the art will appreciate, a normaloperational mode is the operational mode of the IMD prior to it beingaltered in presence of EMI. Thus, for cardiac rhythm management devices(“CRM”), such as Brady and/or Tachy devices, for example, the normaloperational mode is the CRM's initially programmed mode.

The term “MRI-safe” mode, as used herein, can refer to any operationalmode of an IMD that is a safe operational mode in the presence of EMI.For example, for a Brady device (as well as a Brady engine in a Tachydevice) an MRI-safe mode might be a fixed-rate and/or non-demand (orasynchronous) pacing mode as opposed to a rate-responsive and/or demandpacing mode. In some embodiments, an MRI-safe mode can be both anon-demand mode (i.e., VOO) and a non-rate-responsive mode. Thus, inaccordance with one embodiment, switching a Brady device to an MRI-safemode might entail switching the Brady engine to a VOO, AOO or DOO pacingmode. The mode to which the device is switched will depend, of course,on the original programmed mode of the device. In one embodiment, adevice, which is normally programmed to a Dxx mode (i.e., DDDR, DDD,DDI, or DVI) would switch to DOO when in MRI-safe mode. Similarly, adevice programmed to Vxx mode would switch to VOO, and a deviceprogrammed to Axx mode would switch to AOO mode.

Further, in other embodiments, an MRI-safe mode for a Tachy device mightcomprise turning-off tachy detection and/or therapy, as well asswitching the Brady engine of the Tachy device to a fixed-rate,non-demand pacing mode. In these embodiments, turning the tachydetection off will ensure that noise that might be induced on the deviceleads by an MRI scan is not mistaken by the device for tachycardia,which might result in an inappropriate shock during an MRI. Also, forCRM devices, there may be other modes of operation that are consideredsafe in an MRI environment, so the present invention is not limited tothe MRI-safe modes discussed herein. Further, as one skilled in the artwill appreciate, other types of IMDs will have different mode types thatmight be considered safe in an MRI environment, and those modes also areconsidered MRI-safe modes for purposes of the present invention.

Referring now to FIG. 1, diagram 100 illustrates an MRI systemenvironment in which it can be beneficial to detect the presence of anIMD and alter the processing of the MRI system and/or the IMD to preventdamage to the IMD and/or the patient with the IMD. This particulardiagram illustrates a patient 110 having an IMD 120 in the presence ofan MRI system 130. In this particular embodiment, MRI system 130includes a telemetry system 140, which is operable to communicatewirelessly (e.g., wireless link 160) with IMDs. Telemetry system 140 canbe integral with MRI system 130, or telemetry system 140 can be aseparate device in communication with MRI system 130, for example, via aUSB connection, a firewire connection, a network, or any other suitablecommunication connection. In addition, as illustrated in diagram 100,IMD 120 further is operable to wirelessly communicate (e.g., wirelessconnection 170) with an external programming device 150, which cancollect information from IMD 120, as well as reprogram IMD 120.

Thus, as discussed in more detail below, in some embodiments, MRI system130 can detected the presence of an IMD 120 (e.g., using telemetrysystem 140), and then prevent MRI scans if the IMD is not in a safe modeof operation. In other embodiments, IMD 120 can be operable to detectthe presence of EMI (for example magnetic and/or RF signal from MRIsystem 130) and then alter its programming (either automatically, ormanually via external programming device 150) to put the IMD in a safemode of operation. In still other embodiments, IMD 120 can be operableto detect the presence of MRI system 130, and then send commands orinformation to MRI system 130, disabling MRI scans until the IMD can beprogrammed into a safe mode of operation. In still other embodiments,IMD 120 can be manually programmed (e.g. via external programming device150) into safe modes of operation prior to being exposed to EMI, such asMRI scans, or the like. The inter-working relationships between IMD 120and MRI system 130, telemetry system 140 and external programming device150 will be discussed in more detail below.

In accordance with the present invention, IMD 120 can be any type ofimplantable medical device that might be affected by EMI, and inparticular, MRI scans. For example, IMD 120 can be a pacemaker, animplantable cardioverter defibrillator (“ICD”), a cardiacresynchronization device, a bi-ventricular pacer, a ventricular assistblood pump, a drug delivery pump, a drug infusion device, aneurostimulating device, an intra-ocular shunt, an intra-cranial shunt,or any other suitable implantable device that might be sensitive to EMI.In the embodiment illustrated in FIG. 1, IMD 120 is a cardiac device,such as a pacemaker, an ICD, or the like.

Referring now to FIG. 2, one embodiment of an IMD 120 is illustrated. Inaccordance with the illustrated embodiment, IMD 120 comprises aprocessor 202, a memory 204, communication circuitry 206, therapycircuitry 208 and sensor circuitry 210, timer circuitry 212, and an EMIdetector 214. In this particular embodiment, memory 204, communicationcircuitry 206, therapy circuitry 208, sensor circuitry 210, timercircuitry 212, and EMI detector 214 all are in electrical communicationwith processor 202, as is illustrated by the arrows in FIG. 2.

The embodiment of IMD 120 illustrated in FIG. 2 is merely one exemplaryembodiment of an IMD. One skilled in the art will appreciate that otherIMDs might include more features or functionality not shown in FIG. 2,or other IMDs might include less features and/or functionality. Forexample, some IMDs might not provide therapy, so therapy circuitry 208might not be present. Further, as discussed below, having both timercircuitry 212 and EMI 214 might not be needed in all embodiments, so IMD120 may be configured without one or both of those features. Thus, thepresent invention is not limited to the IMD illustrated in FIG. 2.

As one skilled in the art will appreciate, processors and memory devicesare well known in the art, and the specific type and/or style ofprocessor or memory device that can be used in IMD 120 is not limited.Accordingly, processor 202 can be any suitable processing devicecurrently known or hereinafter developed, and memory device 204 can beany suitable memory device currently known or hereinafter developed.

Communication circuitry 206 is circuitry that allows IMD 120 tocommunicate with other devices, such as external programming device 160,telemetry system 140, other IMDs, or other external devices. Asdiscussed above, IMD 120 can communicate with other devices via awireless connection. The wireless connection can be, for example, a nearfield radio frequency (RF) communication connection, a far field RFcommunication connection, an acoustic communication connection (e.g., anultrasound connection), an optical communication connection, or anyother suitable wireless communication connection.

In one embodiment, communication circuitry 206 can include circuitry forboth near field RF telemetry and far field RF telemetry. For example,various embodiments of communication circuitry that can be used in IMD120 are disclosed in Published U.S. Patent App. No. US 2003/0114897 A1,published on Jun. 19, 2003, and entitled “Implantable Medical Devicewith Two or More Telemetry Systems,” and Published U.S. Patent App. No.U.S. 2003/0114898 A1, published on Jun. 19, 2003, and entitled“Telemetry Duty Cycle Management System for an Implantable MedicalDevice,” both of which are incorporated by reference herein for allpurposes.

In addition, in other embodiments, power saving wireless communicationcircuitry and methods can be used. For example, the IMD communicationcircuitry 206 can be configured to reside in a power-saving, sleep modefor a majority of the time. In accordance with this embodiment,communication circuitry 206 can be configured to “wake-up” on a periodicbasis to communicate with an external device. Upon “wake-up” theexternal device will monitor for RF activity, and if the external devicelocates it, communication between the IMD and the external device can beinitiated. There are a number of different ways IMD power-saving modescan be implemented, and the present invention is not limited to anyparticular one. Indeed, the aforementioned Published U.S. Patent App.Nos. US 2003/0114897 A1 and US 2003/0114898 A1 disclose different waysof implementing IMD power-saving modes, which, as discussed above, areincorporated herein by reference for all purposes. In addition,additional power management systems and methods are disclosed inPublished U.S. Patent App. No. US 2003/0149459 A1, published on Aug. 7,2003, and entitled “Methods and Apparatuses for Implantable MedicalDevice Telemetry Power Management” and Published U.S. Patent App. No. US2002/0147388 A1, published on Oct. 10, 2002, and entitled “PassiveTelemetry for Implantable Medical Device,” both of which areincorporated by reference herein for all purposes.

Further, in accordance with other embodiments, communication circuitry206 can be configured to communicate with an intermediary telemetrydevice, which, in turn, can facilitate communication with the externalmonitoring device 104 and/or external computing device 106. One exampleof this type of configuration is disclosed in Published U.S. Patent App.No. US 2003/0130708, published on Jul. 10, 2003, and entitled “Two-HopTelemetry Interface for Medical Device,” the entirety of which isincorporated by reference herein for all purposes. In addition, otherconfigurations for RF telemetry are known, and communication circuitry206 can embody those configurations, as well. Thus, as one skilled inthe art will appreciate, communication circuitry 206 is not limited byany particular configuration or communication means.

Therapy circuitry 208 comprises circuitry for providing one or moretherapeutic functions to a patient. For example, therapy circuitry 208can include circuitry for providing heart pacing therapy, cardiacdefibrillation therapy, cardiac resynchronization therapy, drug deliverytherapy, or any other therapy associated with a suitable IMD. In thecase of cardiac therapy (e.g., pacing, defibrillation, etc.), therapycircuitry 208 can include cardiac leads for delivering the therapy toparticular locations in the heart. In other embodiments, the therapycircuitry and/or therapy delivery mechanisms can reside in a satellitedevice wirelessly coupled to the IMD body 120, as discussed below.

Sensor circuitry 210 comprises the sensors and circuitry needed toobtain or measure one or more physiologic parameters. For example, toobtain a blood pressure (e.g., intravascular or intracardiac bloodpressure), sensor circuitry 210 comprises one or more pressure sensorsand associated circuitry for recording the pressure accurately. Pressuresensors and the associated circuitry are well known in the art, andtherefore, will not be disclosed in detail herein. In addition, in otherembodiments, sensor circuitry 210 can be configured to obtain otherphysiologic parameters, such as temperature, electrical impedance,position, strain, pH, fluid flow, blood oxygen levels, and the like. Inthese cases, sensor circuitry 210 will include suitable bio-sensors forobtaining the corresponding physiologic parameters. Also, as one skilledin the art will appreciate, the sensors and/or sensor circuitry can be,and many times are, electrically coupled to IMD 120, but placed remotelyfrom the IMD; e.g., at the end of a lead or in a satellite device inwireless communication with IMD 120.

In an alternative embodiment, IMD 120 can comprise a planet/satelliteconfiguration, in which the satellite portion of the IMD includes sensorand/or therapy delivery circuits and mechanisms. Such a configuration isdisclosed in Published U.S. Patent Application No. US 2003/0158584 A1,published on Aug. 21, 2003, and entitled “Chronically-Implanted Devicefor Sensing and Therapy,” the entirety of which is incorporated hereinby reference for all purposes. In this system, the planet or main bodyof the IMD communicates with one or more satellite sensor/therapydevices either by an electrical wire connection or wirelessly. In someembodiments, the planet or main body can command each satellite toprovide sensing functions and therapy functions, such as deliveringcardiac electrical pulses, drug delivery, or other functions, asdiscussed above. In other embodiments, the satellite devices canfunction autonomously, and then communicate with the planet device attheir own direction, at the planet's direction, or at timed intervals.The relationships between the planet device and the satellite device(s)are discussed in more detail in the incorporated reference.

Timer circuitry 212 can comprise any suitable circuitry and/orfunctionality for tracking time periods. Timer circuitry can be aseparate timer circuit, as illustrated in FIG. 2, or the timingfunctionality can be performed, for example, by processor 202. As oneskilled in the art will appreciate, timers are well known in the art,and the particular circuitry that performs the timing functionality isnot important. Thus, the present invention is not limited to anyparticular timer embodiment. The use of a timer and/or timer circuitry212 will be discussed in greater detail below.

Finally, EMI detector 214 can comprise one or more detectors fordetecting electro-magnetic fields and/or radiation. For example, EMIdetector 214 can include a sensor for detecting the presence and/orstrength of magnetic fields, such as a Hall-effect sensor, or othersuitable magnetic field detectors currently known or hereinafterdeveloped. In addition, EMI detector 214 can further include sensors fordetecting the presence of high-frequency radiation that can be producedby MRI systems, radar, radio transmitters, and the like. Again, thepurpose and use of EMI detector 214 will be discussed in greater detailbelow.

Referring now to FIG. 3, one embodiment of a telemetry system 140 thatcan be associated with MRI system 130 is shown. As discussed above,telemetry system 140 is operable to communicate with IMDs that might benear MRI system 130. In the illustrated embodiment, telemetry system 140comprises a processor 300, an RF transmitter 310, and RF receiver 320, atransmit/receive (“T/R”) switch 330 and an antenna 340. In thisparticular embodiment, processor 300 is interfaced to RF transmitter 310and RF receiver 320, both of which are connected to antenna 340. T/Rswitch 330 passes RF signals uni-directionally from transmitter 310 toantenna 340 and from antenna 340 to the receiver 320. To communicatedata to an IMD, processor 300 sends data to transmitter 310, whichgenerates an RF carrier signal for a specified time period that isemitted from the antenna 340. As one skilled in the art will appreciate,the carrier signal includes the data to be transmitted to the IMD. Thetransmitted carrier signal then reaches the IMD, which, in turn,receives and processes the data. Similarly, when communicating data orinformation from the IMD to telemetry circuitry 140, the communicationcircuitry 206 of the IMD is operable to generate and transmit an RFcarrier signal to antenna 340. After reaching antenna 340, the carriersignal is conveyed through T/R switch 330 to receiver 320, where thesignal is demodulated to extract the digital message data. The digitaldata may then be processed and interpreted by software executed by theprocessor 300.

T/R switch 330 of the telemetry circuitry enables receiver 320 toreceive signals without having to recover from saturation from signalsthat were previously emitted by antenna 340 that originate fromtransmitter 310. As an alternative to the T/R switch, a directionalcoupler could be used to separate the transmit and receive signals, orseparate antennas with orthogonal linear polarization states can beprovided for the transmitter and receiver, thus enabling simultaneousradiation of the carrier signal by the transmitter antenna and receptionof the reflected carrier by the receiver antenna.

A more complete description of near-field and far-field telemetry is setforth in the patents incorporated by reference above. As one skilled inthe art will appreciate, the present invention is not limited to anyspecific telemetry circuitry or functionality.

Referring again to FIG. 1, external programming device 150 can be anysuitable computing device adapted to communicate with IMD 120 and/ortelemetry circuitry 140 and process data from those devices. Forexample, in the case of a cardiac rhythm management (“CRM”) IMD (e.g.,pacemaker, ICD, etc.), external programming device 150 might be aprogrammer used by physicians, specialists, or other health careproviders to extract data from and program cardiac IMDs. Programmers arewell known in the art. In addition, in other embodiments, externalprogramming device 150 can be a repeater device associate with apatient. Examples of one or more repeater-type devices are disclosed inU.S. Pat. No. 6,607,485, issued on Aug. 9, 2003, and entitled “ComputerReadable Storage Medium Containing Code for Automated Collection andAnalysis of Patient Information Retrieved from an Implantable MedicalDevice for Remote Patient Care,” the entirety of which is incorporatedby reference herein for all purposes.

Referring now to FIG. 4, flow chart 400 illustrates one embodiment of amethod for programming an IMD in a safe mode of operation while in thepresence of MRI systems or other EMI. In accordance with the methodillustrated in flow chart 400, an IMD (e.g., IMD 120 in FIG. 1) receivesa command from an external programming device to switch to a safe modeof operation (e.g., an MRI-safe mode as discussed above) (block 402).The external programming device could be external programming device150, or the external programming device could be associated with MRIsystem 130 and could communicate with the IMD via a telemetry interface,such as telemetry system 140.

Upon receiving a command to switch to a safe mode of operation, aprocessor within the IMD (e.g., processor 202 of IMD 120) will programthe IMD's operational mode to safe mode (block 404), and then a timerwithin the IMD (e.g., timer 212 of IMD 120) will begin measuring a timeperiod from when the reprogram occurs (block 406). When the MRI or otherEMI exposure is complete, the IMD can be manually programmed back to anormal mode of operation by sending it a command to do so. In accordancewith this particular embodiment, the purpose of the timer is to ensurethat the IMD does not remain in the safe mode of operation for extendedperiods of time (e.g., should the operator forget to send the manualcommand to return the device to normal mode), because generally, it isin the patient's best interest to have the IMD operating in its normalmode of operation as originally programmed. The IMD's safe mode ofoperation should be limited to time periods when the IMD is in thepresence of EMI, such as MRI scans, and the like. Thus, after the timerreaches a predetermined time (e.g., a time period after an MRI scan iscomplete), if the IMD has not received a command to switch back tonormal mode of operation, the IMD is switched back from the safe mode ofoperation to its normal mode of operation (block 408). As discussedabove, the processor within the IMD can be operable to reprogram theIMD's operation mode switch.

Referring now to FIG. 5, flow chart 500 illustrates another embodimentof a method for programming an IMD in a safe mode of operation while inthe presence of MRI systems or other EMI. In accordance with the methodillustrated in flow chart 500, an IMD (e.g., IMD 120 in FIG. 1) receivesa command from an external programming device to reprogram to anMRI-mode of operation (block 502). The external programming device couldbe external programming device 150, or the external programming devicecould be associated with MRI system 130 and could communicate with theIMD via a telemetry interface, such as telemetry system 140.

In accordance with this particular embodiment of the invention, theMRI-mode of operation is not a “safe mode” in which a Brady device isset to a fixed-rate, non-demand pacing mode or tachy detection ortherapy of a tachy device is disabled, as discussed above. In accordancewith this particular embodiment, the MRI-mode of operation is a pre-MRIscan setting in which a magnetic field detector is activated (e.g., EMIdetector 214 of IMD 120 in FIG. 2). Thus, when the magnetic fielddetector of the IMD detects a magnetic field of sufficient strength(i.e., in the presence of an MRI system) (block 504), the IMD willautomatically switch to a safe mode of operation (block 506), as definedabove. Once in safe mode, the magnetic field detector of the IMD willcontinue to monitor for the presence and/or absence of the magneticfield (block 508). When the magnet field has dissipated to safe level,the IMD will automatically switch back to its normal mode of operation(block 510)

Further, as one skilled in the art will appreciate, it can be unsafe tohave an IMD operating in the MRI-mode of operation for long periods oftime, because the IMD will automatically switch to a safe mode in thepresence of magnetic fields, even when it is not necessary or desirableto have the IMD in the safe mode. Thus, in accordance with thisparticular embodiment, after an MRI scan is complete or after the IMD ispositioned a safe distance from strong EMI, the IMD can be taken out ofthe MRI-mode of operation by receiving a command from an externalprogramming device and processing the command (block 512).

Referring now to FIG. 6, flow chart 600 illustrates an embodiment of amethod for operating an IMD in a safe mode of operation while in thepresence of EMI. In accordance with the method illustrated in flow chart600, an IMD (e.g., IMD 120 in FIG. 1) includes an EMI detector (EMIdetector 214 in FIG. 2), which is operable to detect EMI, such asmagnetic fields and/or RF energy (block 602). Thus, when the magneticfield detector of the IMD detects a magnetic field of sufficientstrength (e.g., in the presence of an MRI system), the IMD willautomatically switch to a safe mode of operation, as defined above(block 604), and then a timer within the IMD (e.g., timer 212 of IMD120) will begin measuring a time period from when the switch to safemode occurs (block 606). As discussed above, the purpose of the timer isto ensure that the IMD does not remain in the safe mode of operation forextended periods of time. Thus, after the timer reaches a predeterminedtime (e.g., a time period after the IMD is a safe distance from theEMI), the IMD is switched back from the safe mode of operation to itsnormal mode of operation (block 608). As discussed above, the processorwithin the IMD can be operable to reprogram the IMD's operation modeswitches.

Referring now to FIG. 7 a, flow chart 700 illustrates another embodimentof a method for programming an IMD in a safe mode of operation while inthe presence of MRI systems or other EMI. In accordance with the methodillustrated in flow chart 700, an IMD (e.g., IMD 120 in FIG. 1) receivesa command from an external programming device to switch to an MRI-modeof operation, which is discussed above (block 702). The externalprogramming device could be external programming device 150, or theexternal programming device could be associated with MRI system 130 andcould communicate with the IMD via a telemetry interface, such astelemetry system 140.

As discussed above, the MRI-mode of operation is a pre-MRI scan settingin which a magnetic field detector is activated (e.g., EMI detector 214of IMD 120 in FIG. 2). Thus, when the magnetic field detector of the IMDdetects a magnetic field of sufficient strength (i.e., in the presenceof an MRI system) (block 704), the IMD will automatically switch to asafe mode of operation (block 706), as defined above. Once in safe mode,a timer within the IMD (e.g., timer 212 of IMD 120) will begin measuringa time period from when the switch to safe mode occurs (block 708). Asdiscussed above, the purpose of the timer is to ensure that the IMD doesnot remain in the safe mode of operation for extended periods of time.Thus, after the timer reaches a predetermined time (e.g., a time periodafter an MRI scan is has completed), the IMD is switched back from thesafe mode of operation to its normal mode of operation (block 710).

After an MRI scan is complete or after the IMD is positioned a safedistance from strong EMI, the IMD can be taken out of the MRI-mode ofoperation (block 712). In one embodiment, the IMD can be taken out ofthe MRI-mode of operation by receiving a command from an externalprogramming device and processing the command, as discussed above withreference to FIG. 5. In an alternative embodiment illustrated in FIG. 7b, instead of using an external programming device to switch the IMD outof MRI-mode, a timer can be used. In this particular embodiment, afterthe IMD is switched back to normal operation mode (block 710), a timerwithin the IMD (e.g., timer 212 of IMD 120) will begin measuring a timeperiod from when the switch to normal mode occurs (block 720). Then,after the timer reaches a predetermined time, the IMD automatically isswitched out of the MRI-mode of operation (block 722), so that it willnot accidentally detect EMI and switch into safe mode again.

Referring now to FIG. 8, flow chart 800 illustrates yet anotherembodiment of a method of operating an IMD in a safe mode of operationwhile in the presence of MRI systems or other EMI. In accordance withthe method illustrated in flow chart 800, an IMD (e.g., IMD 120 in FIG.2) is operable to measure one or more electromagnetic field componentsgenerated by an MRI system, such as the strength of a static magneticfield (block 802), and/or the amplitude of RF signals at a predeterminedfrequency (block 804). In accordance with this aspect of the invention,the IMD can comprise one or more detectors for detecting the staticmagnetic field and/or the RF amplitude. In one embodiment, a single EMIdetector (e.g., EMI detector 214 in FIG. 1) can be used to measure boththe magnetic field and/or the RF amplitude. In an alternativeembodiment, a hall effect sensor can be used to measure the magneticfield, and an RF sensor or detector can be used to measure the amplitudeof the RF signals. As one skilled in the art will appreciate, a bandpass or notch filter can be used to select the frequency at which theamplitude of the RF signal is measured. In one embodiment, the RF signalamplitude can be measured at about 64 MHz. In other embodiments, the RFsignal can be measured at other frequencies.

In one embodiment, measuring the magnetic field and the RF amplitude canprovide redundant measurements, so that an IMD will not switch to safemode unless both conditions are met (if both conditions are met, the IMDmost likely is near an MRI system). Thus, if the magnetic field exceedsa predetermined strength threshold (e.g., about 0.001 Tesla, or so),and/or the RF signal exceeds an amplitude threshold (e.g., about 0.2 mTper meter) at the particular frequency, the IMD automatically willswitch to a safe mode of operation, as defined above (step 806). Once insafe mode, the IMD will continue to monitor the magnetic field and/orthe RF signal amplitude at the predetermined frequency (blocks 808 and810). When the magnetic field has dissipated to a safe level (e.g.,below 0.001 T or less than 1% of the full field strength) and/or the RFsignal amplitude drops, the IMD will automatically switch back to itsnormal mode of operation (block 812). While this particular embodimentuses both the static magnetic field strength and RF amplitudemeasurements, one skilled in the art will appreciate that otherembodiments might only measure and use one of the measurements, or otherelectromagnetic field components can be used. Thus, the presentinvention is not limited to any one particular embodiment.

Referring now to FIG. 9, flow chart 900 illustrates yet anotherembodiment of a method of operating an IMD in a safe mode of operationwhile in the presence of MRI systems or other EMI. In accordance withthe method illustrated in flow chart 900, an IMD is operable to measurethe strength of a static magnetic field generated by an MRI system(block 902). In addition, the IMD is operable to measure the amplitudeof RF signals generated by the MRI system at a predetermined frequency(block 904). These steps are similar to steps discussed above withreference to FIG. 8. Again, if the magnetic field exceeds apredetermined strength threshold, and the RF signal exceeds an amplitudethreshold at the particular frequency, the IMD automatically will switchto a safe mode of operation (step 906). Next, the IMD sets apredetermined time period and starts a timer (block 908). The IMD thenwill continue to monitor the magnetic field and/or the RF signalamplitude at the predetermined frequency (blocks 910). If the magneticfield continues to exceed the predetermined strength and the RF signalcontinues to exceed the predetermined amplitude, the predetermined timeperiod is extended by an incremental amount (block 912). Otherwise, thetime period is not extended. Next, the timer is checked to determine ifit has reached or passed the predetermined time period (decision block914). If not, steps 910-914 are repeated. If the timer has reached orpassed the predetermined time period, then the IMD will automaticallyswitch back to its normal mode of operation (block 916).

Referring now to FIG. 10, flow chart 1000 illustrates yet anotherembodiment of a method for safely operating an IMD in the presence of anMRI system. In accordance with this particular embodiment, the MRIsystem (e.g., MRI system 130 in FIG. 1) detects the presence of an IMD(block 1002). As discussed above, a telemetry system (e.g., telemetrysystem 140 in FIG. 1) associated with the MRI system can detect IMDsusing wireless communications, such as near field and/or far field RFtelemetry. Alternatively, in another embodiment, instead of thetelemetry system detecting the presence of the IMD, the IMD can beoperable to detect the presence of an MRI magnetic field and thencommunicate with the telemetry system associated with the MRI system,informing the MRI system that the IMD is present.

Once the MRI System and the IMD initiate communications, the MRI systemcan receive information about the IMD via the telemetry link. In oneembodiment, the MRI system receives at least some data from the IMDindicating whether the IMD is in a safe mode of operation, as definedabove (block 1004). If the IMD is in a safe mode of operation (decisionblock 1006), the MRI system can proceed with an MRI scan (block 1008).Alternatively, if the IMD is not in a safe mode of operation, one ormore functions may occur (which is illustrated as alternative block1010).

In one embodiment, if the IMD is not in a safe mode of operation, theMRI system or the telemetry system associated with the MRI system cansend an alarm message to the MRI operator, informing the operator that anon-safe IMD is present (block 1012). Alternatively, in anotherembodiment, instead of sending an alarm message to the MRI operator, theMRI system can be operable to automatically prevent MRI scans fromoccurring when an IMD is present, but not in a safe mode of operation(block 1012). In yet another embodiment, the MRI system can be operableto send an alarm and disable MRI scan functionality.

In yet another embodiment of the invention, if an IMD is not in a safemode of operation, the MRI system and/or the telemetry system associatedwith the MRI can transmit a command wirelessly to the IMD instructing itto switch to a safe mode of operation (block 1014). After the IMDswitches to a safe mode of operation, the MRI system then can conduct anMRI scan (1016). In some embodiment, the telemetry system and/or the MRIsystem will wait for a message from the IMD confirming that the IMDswitched to a safe mode of operation prior to conducting the MRI scan.Upon completion of the MRI scan, the MRI system, then can send a commandto the IMD instructing it to switch back to its normal mode ofoperation, which the IMD will do upon receiving the command (block1018).

In conclusion, the present invention provides novel systems, methods anddevices for mitigating the hazards associated with using IMDs in thepresence of EMI, and in particular, in MRI environments. While detaileddescriptions of one or more embodiments of the invention have been givenabove, various alternatives, modifications, and equivalents will beapparent to those skilled in the art without varying from the spirit ofthe invention. Therefore, the above description should not be taken aslimiting the scope of the invention, which is defined by the appendedclaims.

1. A method for causing an implantable medical device (“IMD”) to operatein a safe mode during an MRI scan, the method comprising: detecting thepresence of the IMD using an MRI system containing a telemetry devicewirelessly communicating with the IMD, the IMD initially programmed tooperate according to a first mode of therapy; determining that the IMDis not operating according to an MRI-safe mode of therapy using thetelemetry device; sending a signal from the telemetry device to the IMDcausing the IMD to operate according to the MRI-safe mode of therapy;and preventing an MRI system from conducting an MRI scan if the IMD isnot in the MRI-safe mode, wherein causing the IMD to operate accordingto the MRI-safe mode of therapy includes at least one of: causing thefirst mode of therapy of the IMD to change from rate-responsive cardiacpacing to fixed-rate cardiac pacing; causing the first mode of therapyof the IMD to change from demand cardiac pacing to non-demand cardiacpacing; causing the first mode of therapy of the IMD to change fromproviding tachycardia therapy to not providing tachycardia therapy; andcausing the first mode of therapy of the IMD to change from tachycardiadetection to not detecting tachycardia.
 2. The method as recited inclaim 1, wherein preventing the MRI system from conducting an MRI scanincludes generating an alarm message for alerting an operator that theIMD is not in the MRI-safe mode.
 3. The method as recited in claim 1,further comprising: disabling the MRI system while the IMD is not in theMRI-safe mode.
 4. The method as recited in claim 1, further comprising:conducting an MRI scan after the IMD is switched to the MRI-safe mode.5. The method as recited in claim 4, further comprising receiving aconfirmation from the IMD that the IMD is in the MRI-safe mode prior toconducting the MRI scan.
 6. The method as recited in claim 1, furthercomprising transmitting a command to the IMD instructing the IMD toswitch back to the first mode of therapy after the MRI scan is complete.7. The method as recited in claim 1, wherein the telemetry device isselected from the group consisting of an IMD programmer and an IMDrepeater device.
 8. The method as recited in claim 1, wherein the IMD isoperable to detect a magnetic field produced by an MRI system, themethod further comprising determining if the IMD is in the MRI-safe modeby receiving a command from the IMD indicating that an IMD is presentand not in the MRI-safe mode.
 9. A method for causing an implantablemedical device (“IMD”) to operate in a safe mode during an MRI scan, themethod comprising: detecting the presence of the IMD using an MRI systemcontaining a telemetry device wirelessly communicating with the IMD, theIMD initially programmed to operate according to a first mode oftherapy; determining that the IMD is not operating according to anMRI-safe mode of therapy using the telemetry device; sending a signalfrom the telemetry device to the IMD causing the IMD to operateaccording to the MRI-safe mode of therapy; receiving a confirmation fromthe IMD that the IMD is in the MRI-safe mode prior to conducting the MRIscan; transmitting a command to the IMD instructing the IMD to switchback to the first mode of therapy after the MRI scan is complete; andpreventing an MRI system from conducting an MRI scan if the IMD is notin the MRI-safe mode; wherein causing the IMD to operate according tothe MRI-safe mode of therapy includes at least one of: causing the firstmode of therapy of the IMD to change from rate-responsive cardiac pacingto fixed-rate cardiac pacing; causing the first mode of therapy of theIMD to change from demand cardiac pacing to non-demand cardiac pacing;causing the first mode of therapy of the IMD to change from providingtachycardia therapy to not providing tachycardia therapy; causing thefirst mode of therapy of the IMD to change from tachycardia detection tonot detecting tachycardia.
 10. The method as recited in claim 9, whereinpreventing the MRI system from conducting an MRI scan includesgenerating an alarm message for alerting an operator that the IMD is notin the MRI-safe mode.
 11. The method as recited in claim 9, furthercomprising: disabling the MRI system while the IMD is not in theMRI-safe mode.
 12. The method as recited in claim 9, further comprising:conducting an MRI scan after the IMD is switched to the MRI-safe mode.13. The method as recited in claim 9, wherein the telemetry device isselected from the group consisting of an IMD programmer and an IMDrepeater device.
 14. The method as recited in claim 9, wherein the IMDis operable to detect a magnetic field produced by an MRI system, themethod further comprising determining if the IMD is in the MRI-safe modeby receiving a command from the IMD indicating that an IMD is presentand not in the MRI-safe mode.